// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

// Based on the XXH128 implementation from https://github.com/Cyan4973/xxHash.

using System;
using System.Buffers.Binary;
using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using static BloomFilter.HashAlgorithms.Internal.XxHashShared;

namespace BloomFilter.HashAlgorithms.Internal;

/// <summary>Provides an implementation of the XXH128 hash algorithm for generating a 128-bit hash.</summary>
/// <remarks>
/// For methods that persist the computed numerical hash value as bytes,
/// the value is written in the Big Endian byte order.
/// </remarks>
internal sealed unsafe class XxHash128 : NonCryptoHashAlgorithm
{
    /// <summary>XXH128 produces 16-byte hashes.</summary>
    private new const int HashLengthInBytes = 16;

    private State _state;

    /// <summary>Initializes a new instance of the <see cref="XxHash128"/> class using the default seed value 0.</summary>
    public XxHash128() : this(0)
    {
    }

    /// <summary>Initializes a new instance of the <see cref="XxHash128"/> class using the specified seed.</summary>
    public XxHash128(ulong seed) : base(HashLengthInBytes)
    {
        Initialize(ref _state, seed);
    }

    /// <summary>Computes the XXH128 hash of the provided <paramref name="source"/> data using the optionally provided <paramref name="seed"/>.</summary>
    /// <param name="source">The data to hash.</param>
    /// <param name="seed">The seed value for this hash computation. The default is zero.</param>
    /// <returns>The XXH128 128-bit hash code of the provided data.</returns>
    public static byte[] Hash(ReadOnlySpan<byte> source, ulong seed = 0)
    {
        byte[] result = new byte[HashLengthInBytes];
        Hash(source, result, seed);
        return result;
    }

    /// <summary>Computes the XXH128 hash of the provided <paramref name="source"/> data into the provided <paramref name="destination"/> using the optionally provided <paramref name="seed"/>.</summary>
    /// <param name="source">The data to hash.</param>
    /// <param name="destination">The buffer that receives the computed 128-bit hash code.</param>
    /// <param name="seed">The seed value for this hash computation. The default is zero.</param>
    /// <returns>The number of bytes written to <paramref name="destination"/>.</returns>
    /// <exception cref="ArgumentException"><paramref name="destination"/> is shorter than <see cref="HashLengthInBytes"/> (8 bytes).</exception>
    public static int Hash(ReadOnlySpan<byte> source, Span<byte> destination, ulong seed = 0)
    {
        if (!TryHash(source, destination, out int bytesWritten, seed))
        {
            ThrowDestinationTooShort();
        }

        return bytesWritten;
    }

    /// <summary>Attempts to compute the XXH128 hash of the provided <paramref name="source"/> data into the provided <paramref name="destination"/> using the optionally provided <paramref name="seed"/>.</summary>
    /// <param name="source">The data to hash.</param>
    /// <param name="destination">The buffer that receives the computed 128-bit hash code.</param>
    /// <param name="bytesWritten">When this method returns, contains the number of bytes written to <paramref name="destination"/>.</param>
    /// <param name="seed">The seed value for this hash computation. The default is zero.</param>
    /// <returns><see langword="true"/> if <paramref name="destination"/> is long enough to receive the computed hash value (8 bytes); otherwise, <see langword="false"/>.</returns>
    public static bool TryHash(ReadOnlySpan<byte> source, Span<byte> destination, out int bytesWritten, ulong seed = 0)
    {
        if (destination.Length >= sizeof(ulong) * 2)
        {
            Hash128 hash = HashToHash128(source, seed);
            WriteBigEndian128(hash, destination);
            bytesWritten = HashLengthInBytes;
            return true;
        }

        bytesWritten = 0;
        return false;
    }

    internal static Hash128 HashToHash128(ReadOnlySpan<byte> source, ulong seed = 0)
    {
        uint length = (uint)source.Length;
        fixed (byte* sourcePtr = &MemoryMarshal.GetReference(source))
        {
            if (length <= 16)
            {
                return HashLength0To16(sourcePtr, length, seed);
            }

            if (length <= 128)
            {
                return HashLength17To128(sourcePtr, length, seed);
            }

            if (length <= MidSizeMaxBytes)
            {
                return HashLength129To240(sourcePtr, length, seed);
            }

            return HashLengthOver240(sourcePtr, length, seed);
        }
    }

    /// <summary>Resets the hash computation to the initial state.</summary>
    public override void Reset()
    {
        XxHashShared.Reset(ref _state);
    }

    /// <summary>Appends the contents of <paramref name="source"/> to the data already processed for the current hash computation.</summary>
    /// <param name="source">The data to process.</param>
    public override void Append(ReadOnlySpan<byte> source)
    {
        XxHashShared.Append(ref _state, source);
    }

    /// <summary>Writes the computed 128-bit hash value to <paramref name="destination"/> without modifying accumulated state.</summary>
    /// <param name="destination">The buffer that receives the computed hash value.</param>
    protected override void GetCurrentHashCore(Span<byte> destination)
    {
        Hash128 current = GetCurrentHashAsHash128();
        WriteBigEndian128(current, destination);
    }

    internal Hash128 GetCurrentHashAsHash128()
    {
        Hash128 current;

        if (_state.TotalLength > MidSizeMaxBytes)
        {
            // Digest on a local copy to ensure the accumulators remain unaltered.
            ulong* accumulators = stackalloc ulong[AccumulatorCount];
            CopyAccumulators(ref _state, accumulators);

            fixed (byte* secret = _state.Secret)
            {
                DigestLong(ref _state, accumulators, secret);
                current = new Hash128(
                    low64: MergeAccumulators(accumulators, secret + SecretMergeAccsStartBytes, _state.TotalLength * Prime64_1),
                    high64: MergeAccumulators(accumulators, secret + SecretLengthBytes - AccumulatorCount * sizeof(ulong) - SecretMergeAccsStartBytes, ~(_state.TotalLength * Prime64_2)));
            }
        }
        else
        {
            fixed (byte* buffer = _state.Buffer)
            {
                current = HashToHash128(new ReadOnlySpan<byte>(buffer, (int)_state.TotalLength), _state.Seed);
            }
        }

        return current;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static void WriteBigEndian128(in Hash128 hash, Span<byte> destination)
    {
        ulong low = hash.Low64;
        ulong high = hash.High64;
        if (BitConverter.IsLittleEndian)
        {
            low = BinaryPrimitives.ReverseEndianness(low);
            high = BinaryPrimitives.ReverseEndianness(high);
        }

        ref byte dest0 = ref MemoryMarshal.GetReference(destination);
        Unsafe.WriteUnaligned(ref dest0, high);
        Unsafe.WriteUnaligned(ref Unsafe.AddByteOffset(ref dest0, new IntPtr(sizeof(ulong))), low);
    }

    private static Hash128 HashLength0To16(byte* source, uint length, ulong seed)
    {
        if (length > 8)
        {
            return HashLength9To16(source, length, seed);
        }

        if (length >= 4)
        {
            return HashLength4To8(source, length, seed);
        }

        if (length != 0)
        {
            return HashLength1To3(source, length, seed);
        }

        const ulong BitFlipL = DefaultSecretUInt64_8 ^ DefaultSecretUInt64_9;
        const ulong BitFlipH = DefaultSecretUInt64_10 ^ DefaultSecretUInt64_11;
        return new Hash128(XxHash64.Avalanche(seed ^ BitFlipL), XxHash64.Avalanche(seed ^ BitFlipH));
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static Hash128 HashLength1To3(byte* source, uint length, ulong seed)
    {
        Debug.Assert(length >= 1 && length <= 3);

        // When source.Length == 1, c1 == source[0], c2 == source[0], c3 == source[0]
        // When source.Length == 2, c1 == source[0], c2 == source[1], c3 == source[1]
        // When source.Length == 3, c1 == source[0], c2 == source[1], c3 == source[2]
        byte c1 = *source;
        byte c2 = source[length >> 1];
        byte c3 = source[length - 1];

        uint combinedl = ((uint)c1 << 16) | ((uint)c2 << 24) | c3 | (length << 8);

        uint combinedh = BinaryHelper.RotateLeft(BinaryPrimitives.ReverseEndianness(combinedl), 13);
        const uint SecretXorL = (unchecked((uint)DefaultSecretUInt64_0) ^ (uint)(DefaultSecretUInt64_0 >> 32));
        const uint SecretXorH = (unchecked((uint)DefaultSecretUInt64_1) ^ (uint)(DefaultSecretUInt64_1 >> 32));
        ulong bitflipl = SecretXorL + seed;
        ulong bitfliph = SecretXorH - seed;
        ulong keyedLo = combinedl ^ bitflipl;
        ulong keyedHi = combinedh ^ bitfliph;

        return new Hash128(XxHash64.Avalanche(keyedLo), XxHash64.Avalanche(keyedHi));
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static Hash128 HashLength4To8(byte* source, uint length, ulong seed)
    {
        Debug.Assert(length >= 4 && length <= 8);

        seed ^= (ulong)BinaryPrimitives.ReverseEndianness((uint)seed) << 32;

        uint inputLo = ReadUInt32LE(source);
        uint inputHi = ReadUInt32LE(source + length - 4);
        ulong input64 = inputLo + ((ulong)inputHi << 32);
        ulong bitflip = (DefaultSecretUInt64_2 ^ DefaultSecretUInt64_3) + seed;
        ulong keyed = input64 ^ bitflip;

        ulong m128High = Multiply64To128(keyed, Prime64_1 + (length << 2), out ulong m128Low);

        m128High += (m128Low << 1);
        m128Low ^= (m128High >> 3);

        m128Low = XorShift(m128Low, 35);
        m128Low *= 0x9FB21C651E98DF25UL;
        m128Low = XorShift(m128Low, 28);
        m128High = Avalanche(m128High);

        return new Hash128(m128Low, m128High);
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static Hash128 HashLength9To16(byte* source, uint length, ulong seed)
    {
        Debug.Assert(length >= 9 && length <= 16);
        ulong bitflipl = (DefaultSecretUInt64_4 ^ DefaultSecretUInt64_5) - seed;
        ulong bitfliph = (DefaultSecretUInt64_6 ^ DefaultSecretUInt64_7) + seed;
        ulong inputLo = ReadUInt64LE(source);
        ulong inputHi = ReadUInt64LE(source + length - 8);
        ulong m128High = Multiply64To128(inputLo ^ inputHi ^ bitflipl, Prime64_1, out ulong m128Low);

        m128Low += (ulong)(length - 1) << 54;
        inputHi ^= bitfliph;

        m128High += sizeof(void*) < sizeof(ulong) ?
            (inputHi & 0xFFFFFFFF00000000UL) + Multiply32To64((uint)inputHi, Prime32_2) :
            inputHi + Multiply32To64((uint)inputHi, Prime32_2 - 1);

        m128Low ^= BinaryPrimitives.ReverseEndianness(m128High);

        ulong h128High = Multiply64To128(m128Low, Prime64_2, out ulong h128Low);
        h128High += m128High * (ulong)Prime64_2;

        h128Low = Avalanche(h128Low);
        h128High = Avalanche(h128High);
        return new Hash128(h128Low, h128High);
    }

    private static Hash128 HashLength17To128(byte* source, uint length, ulong seed)
    {
        Debug.Assert(length >= 17 && length <= 128);

        ulong accLow = length * Prime64_1;
        ulong accHigh = 0;

        switch ((length - 1) / 32)
        {
            default: // case 3
                Mix32Bytes(ref accLow, ref accHigh, source + 48, source + length - 64, DefaultSecretUInt64_12, DefaultSecretUInt64_13, DefaultSecretUInt64_14, DefaultSecretUInt64_15, seed);
                goto case 2;
            case 2:
                Mix32Bytes(ref accLow, ref accHigh, source + 32, source + length - 48, DefaultSecretUInt64_8, DefaultSecretUInt64_9, DefaultSecretUInt64_10, DefaultSecretUInt64_11, seed);
                goto case 1;
            case 1:
                Mix32Bytes(ref accLow, ref accHigh, source + 16, source + length - 32, DefaultSecretUInt64_4, DefaultSecretUInt64_5, DefaultSecretUInt64_6, DefaultSecretUInt64_7, seed);
                goto case 0;
            case 0:
                Mix32Bytes(ref accLow, ref accHigh, source, source + length - 16, DefaultSecretUInt64_0, DefaultSecretUInt64_1, DefaultSecretUInt64_2, DefaultSecretUInt64_3, seed);
                break;
        }

        return AvalancheHash(accLow, accHigh, length, seed);
    }

    private static Hash128 HashLength129To240(byte* source, uint length, ulong seed)
    {
        Debug.Assert(length >= 129 && length <= 240);

        ulong accLow = length * Prime64_1;
        ulong accHigh = 0;

        Mix32Bytes(ref accLow, ref accHigh, source + (32 * 0), source + (32 * 0) + 16, DefaultSecretUInt64_0, DefaultSecretUInt64_1, DefaultSecretUInt64_2, DefaultSecretUInt64_3, seed);
        Mix32Bytes(ref accLow, ref accHigh, source + (32 * 1), source + (32 * 1) + 16, DefaultSecretUInt64_4, DefaultSecretUInt64_5, DefaultSecretUInt64_6, DefaultSecretUInt64_7, seed);
        Mix32Bytes(ref accLow, ref accHigh, source + (32 * 2), source + (32 * 2) + 16, DefaultSecretUInt64_8, DefaultSecretUInt64_9, DefaultSecretUInt64_10, DefaultSecretUInt64_11, seed);
        Mix32Bytes(ref accLow, ref accHigh, source + (32 * 3), source + (32 * 3) + 16, DefaultSecretUInt64_12, DefaultSecretUInt64_13, DefaultSecretUInt64_14, DefaultSecretUInt64_15, seed);

        accLow = Avalanche(accLow);
        accHigh = Avalanche(accHigh);

        uint bound = ((length - (32 * 4)) / 32);
        if (bound != 0)
        {
            Mix32Bytes(ref accLow, ref accHigh, source + (32 * 4), source + (32 * 4) + 16, DefaultSecret3UInt64_0, DefaultSecret3UInt64_1, DefaultSecret3UInt64_2, DefaultSecret3UInt64_3, seed);
            if (bound >= 2)
            {
                Mix32Bytes(ref accLow, ref accHigh, source + (32 * 5), source + (32 * 5) + 16, DefaultSecret3UInt64_4, DefaultSecret3UInt64_5, DefaultSecret3UInt64_6, DefaultSecret3UInt64_7, seed);
                if (bound == 3)
                {
                    Mix32Bytes(ref accLow, ref accHigh, source + (32 * 6), source + (32 * 6) + 16, DefaultSecret3UInt64_8, DefaultSecret3UInt64_9, DefaultSecret3UInt64_10, DefaultSecret3UInt64_11, seed);
                }
            }
        }
        Mix32Bytes(ref accLow, ref accHigh, source + length - 16, source + length - 32, 0x4F0BC7C7BBDCF93F, 0x59B4CD4BE0518A1D, 0x7378D9C97E9FC831, 0xEBD33483ACC5EA64, 0 - seed);

        return AvalancheHash(accLow, accHigh, length, seed);
    }

    private static Hash128 HashLengthOver240(byte* source, uint length, ulong seed)
    {
        Debug.Assert(length > 240);

        fixed (byte* defaultSecret = &MemoryMarshal.GetReference(DefaultSecret))
        {
            byte* secret = defaultSecret;
            if (seed != 0)
            {
                byte* customSecret = stackalloc byte[SecretLengthBytes];
                DeriveSecretFromSeed(customSecret, seed);
                secret = customSecret;
            }

            ulong* accumulators = stackalloc ulong[AccumulatorCount];
            InitializeAccumulators(accumulators);

            HashInternalLoop(accumulators, source, length, secret);

            return new Hash128(
                low64: MergeAccumulators(accumulators, secret + SecretMergeAccsStartBytes, length * Prime64_1),
                high64: MergeAccumulators(accumulators, secret + SecretLengthBytes - AccumulatorCount * sizeof(ulong) - SecretMergeAccsStartBytes, ~(length * Prime64_2)));
        }
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static Hash128 AvalancheHash(ulong accLow, ulong accHigh, uint length, ulong seed)
    {
        ulong h128Low = accLow + accHigh;
        ulong h128High = (accLow * Prime64_1)
                      + (accHigh * Prime64_4)
                      + ((length - seed) * Prime64_2);
        h128Low = Avalanche(h128Low);
        h128High = 0ul - Avalanche(h128High);
        return new Hash128(h128Low, h128High);
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static void Mix32Bytes(ref ulong accLow, ref ulong accHigh, byte* input1, byte* input2, ulong secret1, ulong secret2, ulong secret3, ulong secret4, ulong seed)
    {
        accLow += Mix16Bytes(input1, secret1, secret2, seed);
        accLow ^= ReadUInt64LE(input2) + ReadUInt64LE(input2 + 8);
        accHigh += Mix16Bytes(input2, secret3, secret4, seed);
        accHigh ^= ReadUInt64LE(input1) + ReadUInt64LE(input1 + 8);
    }

    [DebuggerDisplay("Low64: {" + nameof(Low64) + "}, High64: {" + nameof(High64) + "}")]
    internal readonly struct Hash128
    {
        public readonly ulong Low64;
        public readonly ulong High64;

        public Hash128(ulong low64, ulong high64)
        {
            Low64 = low64;
            High64 = high64;
        }
    }
}